U.S. Pat. No. 2,971,853 discloses the production of glass-ceramic articles through the photothermal nucleation and crystallization of precursor glass articles having compositions within the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system. The base glass compositions consist essentially, expressed in terms of weight percent on the oxide basis, of 5.5-15% Li.sub.2 O, 2-25% Al.sub.2 O.sub.3, the weight ratio Al.sub.2 O.sub.3 :Li.sub.2 O being less than 1.7:1, and 60-85% SiO.sub.2, with a photosensitive metal in the indicated proportion selected from the group of 0.001-0.03% gold, computed as Au, 0.001-0.3% silver, computed as AgCl, and 0.001-1% copper, computed as Cu.sub.2 O, the above components constituting at least 90% by weight of the total compositions. The method for producing the articles comprises five general steps:
(1) a glass forming batch of a proper formulation is melted;
(2) the melt is cooled to a glass and essentially simultaneously an article of a desired configuration shaped therefrom;
(3) at least a portion of the glass article is exposed to short wave radiations, customarily ultraviolet radiation;
(4) at least the exposed portion of the article is heated to a temperature between the annealing and softening points of the glass to first initiate the formation of submicroscopic particles of metallic gold, silver, and/or copper to provide nuclei and thereafter to develop siliceous crystallites, including lithium metasilicate, on those nuclei; and then
(5) at least that portion of the article is heated to above the softening point of the glass and to above 800.degree. C., but not above 950.degree. C., to effect an essentially crystalline article, including the conversion of lithium metasilicate crystals to lithium-containing crystals having a lower weight percentage of lithium, e.g., lithium disilicate.
The articles demonstrate the capability of being selectively etched or chemically sculptured. Thus, there is great solubility differential between the crystal phase and the precursor glass. Accordingly, patterns and designs can be cut into the parent glass article through selective exposure of a portion thereof to short wave radiations followed by the heat treatment to generate crystallization therein. Hence, the exposure to short wave radiation develops a latent image in the glass which is converted to a crystallized image via heat treatment. The crystallized areas are many times more soluble in dilute hydrofluoric acid than is the glass. Accordingly, intricate designs can be accurately carved in the article through removal of the crystalline regions.
The patent notes the occurrence of a number of possible crystal phases in the products including lithium disilicate, lithium metasilicate, quartz, .beta.-quartz, and .beta.-spodumene; and the product of one of the working examples provided in the specification, containing lithium disilicate as the major crystal phase with a minor proportion of quartz, exhibited a coefficient of thermal expansion of about 100.times.10.sup.-7 /.degree.C. Also, Corning Glass Works, Corning, N.Y., markets a product prepared in accordance with the method and composition of that patent under the trademark FOTOCERAM.RTM.. That product, designated as Code 8603, has a microstructure consisting essentially of lithium disilicate, exhibits a coefficient of thermal expansion of about 105.times.10.sup.-7 /.degree.C. over the range of temperatures 20.degree.-400.degree. C., and has the following approximate composition, in weight percent:
______________________________________ SiO.sub.2 79.8 ZnO 1.0 Li.sub.2 O 9.4 Sb.sub.2 O.sub.3 0.4 K.sub.2 O 4.0 CeO.sub.2 0.012 Al.sub.2 O.sub.3 3.9 Ag 0.12 Na.sub.2 O 1.5 Au 0.0013 ______________________________________
U.S. Pat. No. 4,328,305 discloses several commercial applications utilizing the capability of Code 8603 articles to be chemically machined; one particular application being head pads for use in the information storage field. Thus, head pads or sliders which support the ferrite element that senses the data magnetically stored on information discs have been fashioned from Code 8603 material. In practice, the glass-ceramic head pad, which contacts the storage disc, is sealed to the ferrite element, customarily through the agency of a commercial sealing frit. Because the ferrite material demonstrates a coefficient of thermal expansion (20.degree.-400.degree. C.) of about 120.times.10.sup.-7 /.degree.C., frequent cracking and breakage have been experienced because of the substantial difference in thermal expansion existing between Code 8603 and the ferrite element. And recently, very high performance manganese-zinc ferrite materials have been developed for the element which exhibit coefficients of thermal expansion up to about 150.times.10.sup.-7 /.degree.C. Accordingly, chemically machinable glass-ceramics manifesting coefficients of thermal expansion substantially higher than Code 8603 were sought, such that seals of low stress could be developed with those ferrites. The production of such glass-ceramic comprises the primary objective of this invention.